Circuit breakers for industrial applications up to 4000 amperes, 600 volts AC, are generally enclosed within switchboard-type compartments which include interlocks to ensure that the breaker contacts are open when the circuit breaker is connected with the power connectors at the rear of the switchboard. U.S. Pat. No. 4,317,160 to Robert S. Tillson et al., which patent is incorporated for purposes of reference, describes one such switchboard. The switchboard compartments are rated in terms of maximum continuous current through the enclosed breakers while the breakers themselves may have varying short circuit current ratings depending upon their application. One approach to increasing the short circuit current capability of a breaker within a particular frame size is to electrically connect a fuse in series with the breaker. The combination of the breaker and fuse rating additively combine to provide substantially increased short circuit protection. When the circuit breaker is contained within one compartment of the switchboard and the fuse is connected in a separate compartment, some means must be provided to assure that the fuses are not withdrawn from the compartment power stabs when the stabs are energized. Since the breaker completes the circuit through the fuses when its contacts are closed, the breaker must always be withdrawn from its compartment power stabs first and connected last so that the fuses are consequently withdrawn from and plugged onto non-energized power stabs.
A known approach to ensuring that the circuit breaker is withdrawn from its compartment power stabs before the fuses and connected with its power stabs after the fuses are connected, is by the use of Kirk keys, which are supplied by the ITE Corporation, Philadelphia, Pa. The switchgear manufacturer arranges a lock on the breaker compartment and a lock on the fuse compartment and provides a single key. When the key is inserted within the breaker compartment lock, the breaker must be withdrawn from the compartment power stabs in order to remove the key from the lock. The key arrangement performs well in most instances, but problems can occur in the event that the single key is lost.
This invention provides a breaker-fuse interlock for ensuring that the circuit breaker is always withdrawn from its compartment power stabs before the fuses are withdrawn and always connected after the fuses are connected. The interlock system of the invention allows the customer to install the breaker and fuse on his own premises, unlike the Kirk key arrangement.